This invention relates to deep ultraviolet catadioptric anamorphic beam expanders, particularly for use in linewidth narrowing in pulsed lasers, including pulsed electric discharge lasers.
Various types of pulsed lasers, including electric discharge lasers, e.g., excimer lasers, and optically pumped dye lasers, typically produce an output beam having a comparatively wide spectral linewidth. For many applications, e.g., microlithography, it is important for the output beam to have a narrow spectral linewidth. To reduce laser spectral linewidth, a number of line narrowing package (LNP) designs have been devised. For example Hansch, xe2x80x9cRepetitively Pulsed Tunable Dye Laser for High Resolution Spectroscopyxe2x80x9d, Applied Optics vol. 11, No. 4,895-898, April 1972, describes a line narrowing package for a dye laser incorporating an intracavity beam expanding telescope together with a diffraction grating mounted in a Littrow arrangement.
It is well known in the art that the wavelength selectivity (or chromatic resolving power) of a diffraction grating is directly proportional to the width of the incident beam on the grating perpendicular to the direction of the grating rulings (see Hansch, cited above). Therefore, in order to maximize the wavelength selectivity of a diffraction grating, a beam expander can be used advantageously. The purpose of a beam expander is to enlarge the beam cross-section to illuminate the entire width of the diffraction grating. Because linear diffraction gratings have dispersive properties in only one axis, it is therefore sufficient to expand the beam in one axis only. Anamorphic beam expansion, which is beam expansion in one axis, can be accomplished by various methods.
A traditional method is to use right angle prisms (see for example Klauminzer, U.S. Pat. No. 4,016,504, issued Apr. 5, 1977. FIG. 1A is a schematic diagram illustrating a current line narrowing package 100 (see Ershov; U.S. Pat. No. 5,852,627, xe2x80x9cLaser with Line Narrowing Output Couplerxe2x80x9d issued Dec. 22, 1998, and incorporated herein by reference in its entirety, which employs right angle prisms 110 for anamorphic expansion of a beam 102. An expanded substantially parallel beam 104 is reflected from a substantially plane mirror surface onto a diffraction grating 108 at a large angle of incidence. Since the purpose of the prisms 110 is to expand beam 102, a maximum amount of expansion per prism is desirable. From Snell""s Law it can be deduced by those skilled in the art that this is readily accomplished with a right angle prism. FIG. 1B is a schematic diagram illustrating the expansion of a beam 122 by a single right angle prism 120. Beam 122 is incident on a prism face 124 at an incidence angle (xcex1, and is refracted through right angle prism 120. An expanded beam 126 exits right angle prism nearly normal to the surface of a leg 128 of right angle prism 120. A limitation of this method is that, in order to produce a significant beam expansion with a small number of prisms, beam 122 must have a large incidence angle xcex1 relative to the face of right angle prism 120. Current designs typically require an incidence angle xcex1 of approximately 74 degrees. At these large angles, antireflective (AR) coatings 130 are difficult to produce. This is especially true for high energy deep ultraviolet (DUV) beams produced by excimer lasers. DUV generally refers to wavelengths shorter than 300 nm and longer than 200 nm. Excimer laser wavelengths of principal relevance to the present invention are 248 nm, 193 nm and 157 nm. Therefore, two of these wavelengths are actually shorter than 200nm and can technically be referred to as EUV (extreme ultraviolet), although the EUV distinction is commonly reserved for wavelengths shorter than 100 nm.
DUV beam expanders are in general difficult to make. Large incidence angle AR coatings and particularly large angle DUV AR coatings are difficult to make for a variety of reasons. First, there are only a few materials that are transparent in the DUV. This limits the material choices to those whose indices of refraction are similar. Therefore to achieve a useful coating, the coating must be made of many layers. Multilayer coatings are more difficult to make because tighter constraints are needed on individual layer thicknesses. Also, because of the DUV wavelength there can be greater absorption in the coating. This can lead to blistering or damage to the coating because of the extremely high energy of the photons. Not only do the coatings have to be effective ( less than 1% Reflective), which dictates multilayer coatings, but the coatings must also be able to withstand high energy pulses for billions of shots.
Another limitation of prism beam expanders is thermal nonuniformity. As light rays propagate through the bulk material of the prisms, optical energy is partially absorbed, thereby heating the prism material. There is a large variation between the actual path lengths that different light rays traverse through a prism. Rays that are incident near an apex of a prism, for example ray 122a, travel a short distance through the prism, whereas rays incident near a leg of the prism, for example ray 122b, propagate a long distance through the prism (see FIG. 1B). This nonuniform propagation of rays through the prisms creates nonuniform thermal gradients inside the prisms, which lead to optical wavefront aberrations. The thermal gradients inside the prisms become increasingly more important as wavelengths decrease and pulse repetition rates increase.
Although heat generated per unit optical pathlength (and therefore per unit volume) is substantially uniform, nonuniform thermal distortion in the prism results from nonuniform cooling, because cooling takes place at the prism surface, which is not equidistant from all points within the prism volume. The surface area per volume changes significantly for differing propagation paths through a prism. For example, a propagation path close to the apex of the prism has greater heat conduction to the surface than does a path closer to a leg of the prism. Additionally, nonuniform temperature distributions are produced by the finite heat transfer time of the prism. Intense short bursts of energy produce significant transient nonuniformities. Excimer lasers are typically operated in bursts of pulses with varying time intervals between bursts. It is observed that the current LNP performs differently, depending on the burst duty cycle (the time interval between consecutive bursts of pulses).
It is therefore desirable in the art to provide a method and system for linewidth narrowing in an electric discharge laser having minimal optical elements and complexity, that eliminates large incidence angles and associated antireflective coatings and that reduces vulnerability to thermal distortion and damage. Additionally, it is desirable in the art to provide an improved general purpose DUV beam expander.
A catadioptric anamorphic beam expanding telescope is described, having optical power in a first axis substantially perpendicular to the beam propagation axis. As it propagates through the telescope, the beam is expanded in the first axis, and is deflected in a plane substantially perpendicular to the first axis by off-axis optical elements, thereby providing unobscured beam expansion. The optical elements of the beam expanding telescope can include reflective, refractive, and combined reflective/refractive elements.
In one embodiment, a catadioptric anamorphic beam expanding telescope includes an off axis convex spheric reflector and an off axis combined reflective/reflective optical element, commonly known as a Mangin mirror. The Mangin mirror includes a refractive first surface and a reflective rear surface, which are configured to compensate for aberrations introduced by the off axis deflection of the beam. The catadioptric anamorphic beam expanding telescope is particularly useful for deep ultraviolet (DUV) applications at wavelengths shorter than about 250 nm. For these applications, refractive elements of the telescope are fabricated advantageously from calcium fluoride (CaF2).
In some applications, the catadioptric anamorphic beam expanding telescope is used to illuminate a diffraction grating or other wavelength dispersive element. A diffraction grating aligned with a catadioptric anamorphic beam expanding telescope retroreflects a preferential wavelength, and thereby provides wavelength narrowing or xe2x80x9cline narrowingxe2x80x9d. In some applications the combined catadioptric anamorphic beam expanding telescope and diffraction grating are incorporated within a laser optical cavity, thereby narrowing the output wavelength spectrum of the laser. Embodiments of the present invention are particularly advantageous for intracavity line narrowing in an electric discharge laser, such as a KrF excimer laser, ArF excimer laser, or F2 molecular laser.
Embodiments in accordance with the present invention provide a method and system using catadioptric, anamorphic optical design for linewidth narrowing of an electric discharge laser. The embodiments involve minimal optical elements and complexity, and avoid difficult and costly a spheric optics, such as conic cylindrical optics. The embodiments eliminate large incidence angles and associated antireflective coatings, and thereby reduce vulnerability to thermal distortion and damage. Additionally, embodiments in accordance with the present invention provide an improved general purpose DUV beam expander.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings.